Although inosine and guanosine can be produced by a fermentation process, on an industrial scale, it is extremely difficult to produce inosine or guanosine alone by fermentation. Then, usually, there is carried out parallel fermentation wherein both nucleosides are accumulated in amounts of more than 1%, respectively.
If 5'-phosphorylation is carried out by using a starting material containing more than 1.5% of guanosine based on inosine, the content of disodium 5'-guanylate in the resulting crystals of disodium 5'-inosinate becomes more than 1% as that in the starting material, unless a special operation is effected.
Therefore, when inosine or guanosine are produced by nucleoside fermentation in order to obtain disodium 5'-inosinate or disodium 5'-guanylate as a final product, it is important to separate the guanosine or inosine which forms as a concomitant product in the fermentation.
Further, when a mixture containing disodium 5'-inosinate and disodium 5'-guanylate is desired in a specific ration in a final product, for example, a product containing equal weights of these two compounds product, the ratio of guanosine/inosine in a fermented fluid should be always held within a certain range. However, it is also extremely difficult to carry out such fermentation.
In conducting the separation of inosine and guanosine from each other, it has been known that inosine is easily soluble in water, while guanosine is almost insoluble in water [solubility described in The Merck Index, 7th ed., 1960, inosine: f1.6 g/100 ml water (20.degree. C.), guanosine: 1 g/1,320 ml water (18.degree. C.), solubility ratio: about 21-fold]. Based on this fact, there have been established known separation processes such as those described in Japanese Patent Publication No. 56-24519 and the like. Basically, these processes utilize the difference between the water solubilities of the nucleosides. For example, in Japanese Patent Publication No. 56-24519, although the pH in the separation step of these two compounds is limited to 2 to 9, there is merely described that, when the pH is outside the above range, a mineral acid salt or alkali salt is formed.
As described above, by using inosine-guanosine mixed crystals obtained from purification of a fermented fluid as a starting material, the technique for separating it into inosine which is substantially free from guanosine and an inosine-guanosine mixture is of importance from an industrial viewpoint. However, according to a conventional method, when a nucleoside mixture having a high weight ratio of guanosine/inosine, for example a ratio of 0.5 or higher, is simply suspended in water, the weight ratio of guanosine to inosine in the solution considerably exceeds 0.01 (see, Experiment 1 hereinafter and FIG. 1). Thus this method is not satisfactory to obtain inosine substantially free from guanosine.